#include <math.h>
#include <stdio.h>
#define NUM 500000

float great_circle(float lon1, float lat1, float lon2, float lat2){
    float radius = 3956.0;
    float pi = 3.14159265;
    float x = pi/180.0;
    float a,b,theta,c;

    a = (90.0-lat1)*(x);
    b = (90.0-lat2)*(x);
    theta = (lon2-lon1)*(x);
    c = acos((cos(a)*cos(b)) + (sin(a)*sin(b)*cos(theta)));
    return radius*c;
}

int main() {
    int i;
    float x;
    for (i=0; i <= NUM; i++)
        x = great_circle(-72.345, 34.323, -61.823, 54.826);
    printf("%f", x);
}
